The present invention relates generally to a Free Space Optical Local Area Network (LAN) and deals more particularly with apparatus and a related method for maintaining the concurrent viability of a bi-directional free space optical path. The invention also deals with apparatus and a related method for automatically reconfiguring the LAN to provide interconnection of a conventional physical LAN and Free Space Optical LAN.
The two most commercially successful methods for arbitrating access to local area network (LANS) are Carrier Sense Multiple Access/Collision Detect (IEEE 802.3) and Token Ring (IEEE 802.5). The LAN topologies used with these protocols are stars (a central hub), rings, strings, or combinations thereof. Typical media used for communication include coaxial cable, shielded or unshielded twisted pair wire, and fiber optic cable. The present invention uses generally directed optical signals transmitted line of sight through free space. A Free Space Local Area Network System is disclosed in pending applications assigned to the same assignee as the present identification U.S. Ser. No. 07/450,404 filed Dec. 14, 1989 and PCT application, entitled "Free Space Local Area Network System" filed Dec. 14, 1990, both of which are incorporated herein by reference and to which applications reference may be made for further details. Radio, another possible method communication via free space, has not been used commercially to date with these protocols.
The most significant advantages of free space over physical media are more flexibility in adding or moving data communication equipment and, in many installations, less cost. Physical media LAN systems have certain limitations that do not arise with free space optical systems and free space optical systems have several operational characteristics that do not arise with physical media systems. This invention discloses means for minimizing the operational problems that are unique to free space optical systems.
The unique aspects of free space optical systems fall into two general categories. The first stems from line of sight communications and the possibility that a link might become obstructed for short intervals and then return to viability when the obstruction is removed. In contrast, when a physical media link becomes impaired, it is usually because it has become disconnected, severed, or broken and the link cannot be returned to viability until specific remedial actions have been taken. The second unique aspect is that establishing a viable free space optical link from point A to point B does not necessarily result in a viable link from point B to point A. A robust link requires that the link from A to B and the link from B to A be viable concurrently.
Applicants are not aware of relevant prior art on free space local area networks, either radio or optical, dealing with the issues of establishing bi-directional viability of communication links for maintaining alternate links on hot standby if an operating link fails. Prior art related to physical media such as cables, wires, or optical fibers teach means for alternate links on hot standby that require either modifications to Medium Access Control (MAC) or Logical Link Control (LLC) levels of the Open System Interconnection (OSI) Reference Model, a central control station, or complex control communication methodologies involving more than two nodes on the network.
The present invention reconfigures free space optical links to provide an alternative communication path automatically and instantly for network information if an operational free space optical link becomes impaired thereby maintaining ongoing flow of network information without interruption. The free space optical links are restored to the original configuration when the impairment is resolved. Link information to control both reconfiguration and restoration is restricted to the two physical ends of an optical link.
The present invention operates at the physical level of the OSI Reference Model, that is, functionality can be independent of the network protocol being used by the networked communication devices and vice versa.
One object of the present invention is to provide means for network communication via free space optical links that conform with IEEE 802.3 or 802.4 or 802.5 or RS-232 or FDDI or other specifications that are proprietary.
Another object of the invention is to provide indications of received optical power to facilitate alignment of a free space optical link.
A further object is to provide indications of a viable free space optical link and to provide indications when the link is not viable.
A further object is to short circuit the data path from the optical transmitter to the optical receiver, or vice versa, of an optical link when the link becomes impaired and to remove the short circuit when impairment is resolved.
A further object of the invention is for nodes to provide indications of operational status to attached communication devices.
A further object of the invention is to provide means for the nodes to alter their operational status on command from attached communication devices.
A further object of the invention is for neighboring nodes with two or more pairs of optical links to provide hot standby communication capability automatically and instantly if the free space optical link between them becomes non-operational.
A further object of the invention is for neighboring nodes with two or more pairs of optical links that have suffered an impairment to the free space optical link between them and have reconfigured, to be restored to the original configuration when the free space optical link becomes unimpaired.
A further object of the invention is to provide means for the nodes to enable insertion of attached communication devices into the network's data path when a free space optical link is viable and to isolate attached communication devices from an optical link when it is not viable.
A further object of the invention is to limit the propagation of link maintenance control signals to the nodes at the two physical ends of free space optical link.